1. Field of the Invention
The present invention relates to a lithographic system, a method for adapting transmission characteristics of an optical pathway within a lithographic system, a semiconductor device, a method of manufacturing a reflective element for use in a lithographic system, and a reflective element manufactured thereby.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction, i.e. the “scanning” direction, while synchronously scanning the substrate parallel or anti-parallel to this direction.
Lithographic apparatus may soon employ EUV radiation sources, e.g. based on a plasma discharge. As opposed to the radiation sources that are currently used in lithographic apparatus having an extremely small bandwidth, i.e. laser sources, these sources are broad-band light sources, i.e. sources which generate radiation of a wide range of wavelengths. In a lithographic apparatus employing EUV radiation, the spectral distribution of the EUV-light emitted by the source is altered by the apparatus. More specifically, the imaging related spectral effects depend on both the transmission properties of optical elements in the lithographic apparatus as well as on the emitted source spectrum. The current knowledge about the detailed spectrum around a wavelength of EUV-light, i.e. a wavelength within a range of 5-20 nm, for example about 13.5 nm, is limited. The current resolution of the spectrum is in the order of 0.05 nm with a 2% bandwidth. Furthermore the real spectrum of the source seems to change by a change in parameters like input power, electrode configuration of the source, emitting material (Xe, Sn or other suitable material) used and history of electrodes of the source due to changes in the plasma.
Due to the lack of knowledge, it is highly probable that an optical pathway, i.e. an optical trajectory of the radiation between its generation by the source and the projection on the target portion of the substrate, within the lithographic apparatus is not optimized for EUV-radiation. The mismatch between the spectrum emitted by the source, and the spectrum expected by the optical components along the optical pathway will lead to errors, for example a chromatical error seen in the apodization i.e. a distortion in an intensity profile. The first order effect of this error will be a telecentricity error and in a later stage a horizontal-vertical bias, i.e. a difference in intensity at the substrate level between light passing horizontal elements and light passing vertical elements within a pattern structure to be exposed, will increase dramatically as is known to those of ordinary skill in the art.